1. Field of the Invention
The present invention relates to a protective-agent applying device, a process cartridge, and an image forming apparatus.
2. Description of the Related Art
In conventional electrophotographic image forming apparatuses, an image is formed by subjecting a photosensitive element to a charging process, an exposure process, a developing process, and a transfer process. Electrical discharge products produced in the charging process remain on the surface of the photosensitive element, and non-transferred toner or toner components also remain on the surface of the photosensitive element after the transfer process. These products and non-transferred toner or toner components are removed from the photosensitive element through a cleaning process.
A rubber blade is generally used for the cleaning process. The rubber blade is inexpensive, simple in mechanism, and excellent in cleaning capability.
However, because the rubber blade removes residual materials from the surface of the photosensitive element as being pressed against it, there is large mechanical stress due to friction between the surface of the photosensitive element and a cleaning blade as the rubber blade. Therefore, the rubber blade is worn, and the surface layer of the photosensitive element or of an organic photosensitive element in particular is worn, which causes both lives of the rubber blade and the organic photosensitive element to be reduced.
Small-sized toner particles are increasingly used for image formation to meet demands for high image quality.
In the image forming apparatus using the small-sized toner particles, residual toner particles often pass through under the cleaning blade. Particularly, when dimensional accuracy of the cleaning blade or the assembly accuracy are insufficient or when the cleaning blade partly vibrates, much more of toner particles pass through under the cleaning blade, resulting in decrease in image quality.
Therefore, to extend the life of the organic photosensitive element and maintain high image quality over a long period, it is necessary to reduce degradation of a material due to friction and improve the cleaning capability.
Japanese Patent Publication No. S51-22380 discloses a technology for employing a method of supplying a metallic soap such as zinc stearate to the photosensitive element and forming a coating of lubricant on the surface thereof by a cleaning blade. This method is preferred because, by using the metallic soap, the lubricating capability on the surface of the photosensitive element is improved and friction between the photosensitive element and the cleaning blade can thereby be reduced. Thus, the cleaning performance can be improved for non-transferred toner.
Recently, on the other hand, alternating current (AC) charging tends to be used for the charging process. The AC charging is performed by using a charging roller or the like that is charged by superimposing an AC voltage on a direct current (DC) voltage.
The AC charging has excellent capabilities such as high uniformity of a charging potential on a photosensitive element, less occurrence of oxidized gas such as ozone and NOx, and minimization of a device. On the other hand, the AC charging has disadvantages such that positive/negative electrical discharge is repeated hundreds to thousands times per second between a charging unit and a photosensitive element according to frequencies of a DC voltage to be applied, which causes degradation of the surface layer of the photosensitive element due to a large number of electrical discharges, to be accelerated. To take a measure against the degradation, by applying the lubricant to the photosensitive element, the energy of the AC charging is first absorbed by the lubricant, so that the energy is difficult to reach the photosensitive element, and the photosensitive element is thereby protected.
The metallic soap is decomposed here by the energy of the AC charging, but this does not mean that the metallic soap is not completely decomposed to disappear. Fatty acid with a low molecular weight is produced at this time, which causes the frictional force between the photosensitive element and the cleaning blade to become high, and the toner components together with the fatty acid are caused to easily adhere to the photosensitive element in a film form. The resolution of an image is thereby easily reduced and at the same time the photosensitive element wears, which easily leads to uneven density. Therefore, a large amount of metallic soap is supplied to the photosensitive element so that the surface of the photosensitive element is immediately coated with the metallic soap even if the fatty acid is produced. However, even if the large amount of metallic soap is supplied to the photosensitive element, only part of the metallic soap actually adheres to the surface of the photosensitive element. Therefore, the most of the metallic soap supplied to the photosensitive element is transferred together with the toner or is removed together with waste toner. This results in early running out of the metallic soap, and thus the metallic soap has to be replaced with new one before the end of useful life of the photosensitive element.
A lubricant as a protective agent instead of the metallic soap is described in, for example, Japanese Patent Application Laid-open No. 2005-274737, in which by using a lubricant supplying device that supplies a lubricant containing higher alcohol as a main component having a carbon number from 20 to 70, the higher alcohol stays at an edge of a blade nip as amorphous particles and this causes the surface of an image carrier (photosensitive element) to become appropriately wet, and thus lubricating capability is continued.
The lubricant based on the higher alcohol is easy to wet the surface of the photosensitive element and the effect as the lubricant can be expected. However, an area occupied by each of higher alcohol molecules absorbed in the image carrier tends to increase, and the density of molecules absorbed in the image carrier per unit area (weight of absorbed molecules per unit area) is low. Consequently, the photosensitive element is difficult to be protected from electrical stress due to the AC charging.
Japanese Patent Application Laid-open No. 2002-97483 describes that by using particular powder of alkylene bis-alkyl acid amide compound as a lubricating component, there exist powdery particles on an interface where a cleaning blade and an image carrier are pressed against each other, which allows smooth lubricating effect to be maintained over a long period of time.
However, the lubricant containing nitrogen atoms in molecules produces an ionic dissociating compound as a decomposed product like a nitrogen oxide and an ammonium-containing compound when the lubricant itself is exposed to the electrical stress due to the AC charging. And the ionic dissociating compound is taken into a lubricant layer and the resistance of the lubricant layer is reduced under high humidity, which may cause image blur to occur.
Furthermore, it is getting clearer that the protective agent containing paraffin as a main component can protect the photosensitive element from the electrical stress due to the AC charging and reduce the frictional force between the photosensitive element and the cleaning blade, and that the cleaning performance of the waste toner becomes extremely better. Particularly, even if the protective agent containing paraffin as a main component is oxidized by the stress due to the AC charging, fatty acid is not produced much, and the frictional force between the photosensitive element and the cleaning blade changes very slightly, which is preferable.
However, when the protective agent containing paraffin as a main component is used to repeatedly form images, an image may sometimes be defective, which is thought due to wear of the photosensitive element and the cleaning blade. Especially, the probability of occurrence of defective images largely changes depending on manufacturing lots of protective-agent applying devices.
Detailed examination was conducted on a location where a defective image was formed and a location where no defective image was formed. As a result, it is clear that the layer thickness of the photosensitive element decreases or much of the toner components adhere to the photosensitive element depending on in the location where a defective image with streaks is formed and in the location where no defective image is formed. However, examiners have no idea about what kind of factor causes these phenomena.
To extend organic photosensitive element life and to maintain high image quality over a long period of time, it is necessary to reduce deterioration of units due to friction and improve cleaning capability. For this purpose, as explained above, the method of supplying the metallic soap such as the zinc stearate to the photosensitive element and forming coating of the lubricant thereon by using the cleaning blade.
By applying the lubricant to the photosensitive element, the surface of the photosensitive element is protected by the lubricant. Thus, wear of the photosensitive element due to the friction between the cleaning blade and the photosensitive element is reduced, and degradation of the photosensitive element due to electrical discharge energy produced when the photosensitive element is charged is also reduced. Furthermore, by applying the lubricant, the lubricating capability of the surface of the photosensitive element is increased, which allows reduction of a phenomenon such that the cleaning blade partly vibrates and reduction of the amount of toner particles passing through under the cleaning blade. However, when the amount of applying the lubricant to the photosensitive element is too little, the lubricating capability and the protection performance are not satisfactorily effective in solving the problems on the wear of the photosensitive element, the degradation of the photosensitive element due to the AC charging, and the pass-through of toner particles. Therefore, the amount of the applied lubricant needs to be specified.
When zinc stearate is used for lubricant, the amount of the zinc stearate applied to the surface of the photosensitive element is evaluated using a ratio of zinc element to all the elements detected by X-ray photoelectron spectroscopy (XPS) analysis of the surface of the photosensitive element. Reference may be had to, for example, Japanese Patent Application Laid-open No. 2005-17469, Japanese Patent Application Laid-open No. 2005-249901, Japanese Patent Application Laid-open No. 2005-004051, and Japanese Patent Application Laid-open No. 2004-198662.
The XPS analysis allows detection of all the elements except for hydrogen on the extreme surface of a sample. Therefore, according to analysis of the surface of an organic photosensitive element with the zinc stearate applied thereto by using the XPS, an element ratio which the organic photosensitive element has is closer to an element ratio which the zinc stearate has with an increase in coverage of the zinc stearate. When the coverage reaches 100%, the element ratio of the organic photosensitive element theoretically coincides with the element ratio of the zinc stearate, and the amount of detected zinc is saturated. More specifically, when the zinc stearate (C36H70O4Zn) covers the entire surface of the photosensitive element, a ratio of the zinc element to all the elements detected by the XPS based on the element ratio in molecules of the zinc stearate (C36H70O4Zn) except for hydrogen becomes 2.44 atomic % in theory.
Therefore, the coverage (Zn/2.44)×100(%) of the zinc stearate can be calculated from atomic % of the zinc. By using the coverage calculated by the amount of zinc in the above manner, it is possible to calculate a favorable coverage of the zinc stearate for improving the wear of the photosensitive element due to the cleaning blade, the pass-through of toner particles, and the degradation due to the AC charging.
A conventional method of evaluating whether application of a protective agent is satisfactory cannot be used depending on a lubricant to be used. More specifically, when a protective agent such as zinc stearate containing metal is used, the amount of metal can be used as an index of the coverage. However, when a protective agent such as paraffin not containing metal is used, peaks of the protective agent detected by the XPS analysis includes only peaks of C and O, and thus the protective agent cannot be separated from the elements contained in the photosensitive element, which makes it difficult to evaluate the amount of the protective agent deposited on the photosensitive element.